1. Field of the Invention
The invention relates to telephone extension line circuitry and techniques for conversion of bi-directional signals on a two wire cable to uni-direction signals for a four wire switching system.
2. Description of the Prior Art
Interface circuits, commonly referred to as "hybrids," are required for connecting conventional telephone sets to telephone extension lines from electronic switching systems such as Private Automatic Branch Exchange (PABX) systems or Telephone Central Office (Level 5) switching circuit systems. The important functions and elements of extension-line circuits include the following. First, high voltage protection devices capable of transferring voice signals to and from the four wire switching system are required to protect the electronic switching system (i.e., the PABX). The two bi-directional signals on the two wire telephone cable must be separated by means of a two-to-four wire converter. A powering circuit is required to feed DC power to the telephone set. The DC feed circuit must be protected against short circuits to the two wire cable. Ring insertion circuitry is required which activates the ringing device in the telephone set when a ring command is sent from the electronic switching system. Finally, a means of status detection is required for indicating changes in the status of the telephone set, such as on-hook and off-hook transitions, etc.
Both the bulk and the cost of electronic switching systems have been greatly reduced as a consequence of the rapid progress in large-scale integrated circuit technology in recent years. However, no significant size reductions have been made in the interface circuits or hybrids which are utilized to connect conventional telephone sets to the improved electronic switching systems. This is because up to now no suitable circuitry has been available to implement the interface circuits without using costly and bulky transformers to achieve high-voltage protection of the electronic switching systems from static discharge surges which are frequently induced on the two wire cable. Transformers simply do not permit high packaging density. The cost of these interface circuits has not been significantly reduced because a number of the functions they must provide, such as high-voltage protection and DC feeding of the two wire telephone cable to power the conventional set, are not compatible with modern integrated circuit technology.
A new approach is needed to overcome the above drawbacks to provide stable, low cost, accurate, transformerless high voltage protected signal transmission and to provide status and ring insertion signaling paths to interface conventional telephones with modern four wire telephone switching systems.
Extension-line circuits must provide a variety of functions to enable connection of conventional telephone sets to digital electronic switching systems using pulse code modulation and time division multiplexing. The necessary analog-to-digital and digital-to-analog conversion in such digital electronic switching systems requires a uni-directional four wire signal system, whereas conventional telephone extension lines require a two wire bi-directional signal system.
Although the hybrid transformer has provided suitable high voltage protection for interface circuits in the past, its bulkiness in relation to modern large scale integrated circuits makes the transformer the limiting component with respect to further size and cost reductions of electronic switching systems. It has been suggested that high voltage isolation problems be approached utilizing optical couplers. However, the emission efficiency of presently available light emitting diodes in optical couplers varies considerably more as a function of time than is acceptable in telephone interface circuits.
Previous interface circuits require isolated high voltage protected DC paths for the signaling functions, including hook status detection, dial pulse detection, and ring insertion control. These functions have been accomplished utilizing expensive relays. These components require additional expense. Electro-mechanical devices such as the previously used relays are inherently much less reliable than modern integrated circuit devices which are used elsewhere in recently developed systems. The transformers and relays are now the limiting factor in size, cost, and reliability of conventional telephone interface circuits.
The conventional resistive DC feed systems of conventional interface circuits utilize two 400 ohm resistors, one coupled between ground and a primary winding of a hybrid transformer and the other connected between a -48 volt battery power source and the other primary winding of the isolation transformer. The cable resistances of each wire of the two wire cable, which may vary between 0 and 500 ohms, is in series with the respective 400 ohm resistors and the 200 ohm resistance of the telephone set when the receiver is off the hook. Consequently, the current fed by the 400 ohm resistors and the battery power source to the telephone set may vary between 24 milliamps and 48 milliamps. Thus, the power consumption may vary by a factor of approximately two and the voltage drop on the usual 200 ohm DC resistance of the telephone set may also vary by a factor of two.
In conventional telephone interface circuits the voice signal from the telephone set is terminated with a 600 ohm impedance. Consequently, the microphone output voltage signal transmitted to the electronic switching system varies as a function of the length (and resistance) of the two wire cable because of voltage division of the microphone output signal between the cable resistance and the 600 ohm termination resistor.
It has been suggested to use modulated current sources to provide the DC feed to the two wire cable and to reproduce the incoming voice signal to provide the signal to be received by the earphone of the telephone set. However, the suggested system utilizes a 600 ohm cable termination, causing attenuation of the microphone output signal as a function of cable length. The previously suggested system uses optical couplers whose signal transmission characteristics are directly proportional to the light emission efficiency of the light emitting diodes therein. These systems are incapable of operating within the required tolerance for telephone extension systems because of the inherent instability of presently available light emitting diodes.
An extension line interface circuit requires low echo return, which occurs when any part of the voice signal received from the electronic switching system is fed back to that four wire electronic switching system as a component of the microphone output transmitted to the electronic switching system. Both the microphone output signal and the voice signal received from the four wire switching system may appear simultaneously on the two wire cable. The component due to the voice signal received from the four wire switching system must be cancelled out before the microphone output signal is transmitted to the four wire switching system. Any part of that component not cancelled out is referred to as "echo return." If a significant echo return is present in the system, the user of the telephone set will hear his own voice louder than normal, and his transmitted voice level will be reduced. In addition, there is a strong possibility that the system will break into oscillation at a particular frequency, causing "singing" which will make that extension line inoperative.
Conventional two-to-four wire converter circuits provide minimum echo return only if they are terminated with the nominal line impedance of 600 ohms. However, variation of line impedance due to cable length is so severe that the echo return signal may be as little as 10 dB below the transmitted signal at either extreme of the line impedance value. Strapping may then be required to maintain the required 14 dB level of the echo return signal below the transmitted signal. This involves additional components and cost.
Ordinarily, expensive high voltage relays are utilized to isolate the interface circuit from the two wire cable to allow a ring voltage to be impressed upon the ringing circuit of the telephone set in response to a ring command from the four wire switching system. The voltage applied to the ringing circuitry is ordinarily approximately 90 volts RMS at 20 H.sub.z and may reach 120 volts in magnitude, so that the high voltage relays are necessary to prevent the ringing signal from damaging the interface circuit. Static discharges on the line which would arc across the relays must be kept out of the switching circuits.
Prior methods of fabricating semiconductor devices utilizing optical couplers and integrated circuits which are operatively connected together usually involves the use of separate packages for two separate integrated circuit chips and use of a special package in which light emitting diodes and photodiodes are housed together to provide optical coupling. Consequently, the packaging and interconnection costs for integrated electronic circuits which utilize optical couplers are substantially higher than for integrated circuits which do not use optical couplers.